Level 4 · Motion · project 13 of 13 · 25 min · ●●●○○
Warm Hand Fan
A little desk fan that starts when you cup your palm over the sensor. Warm-Hand Alarm's brain, motor mouth.
new here? Start with First Light — one battery, one resistor, one LED. It teaches the loop, LED direction, and why the 220 Ω resistor matters.
What you need
- 1× 3×AA battery pack (4.5 V) L1
- 1× Solderless breadboard L1
- 7× Jumper wires L1
- 1× Temperature sensor (10 kΩ thermistor) the "hand sensor" — 10 kΩ at room, drops toward 3.6 kΩ under a warm palm L3
- 1× 1 kΩ resistor divider bottom — makes the threshold sharp L1
- 1× 220 Ω resistor base resistor — smaller than Warm-Hand Alarm because a motor drinks more than an LED L1
- 1× NPN transistor (2N2222) flat face toward you: legs are E-B-C L3
- 1× DC motor (3 V) the little hobby motor — a paper blade makes the breeze visible
- 1× Diode (1N4001) flyback protection across the motor, cathode toward battery +
you'll learn: Voltage dividers · Transistor switching · Ohm's law
Circuit map
The circuit you'll build, running on the Workbench right now — shown with a warm hand on the sensor.
simulating…
If it doesn't work
The boring ways this circuit actually breaks — check these first before you unwire everything.
- Fan doesn't spin, even with a firm warm palm. Check the base resistor first — it must be 220 Ω here, not the 1 kΩ from Warm-Hand Alarm. Then the 2N2222 pins E-B-C flat-face-forward. Then the diode band faces +. With warm hand the middle-node voltage should sit around 0.75 V; anything under 0.7 V means the divider is off.
- Fan runs at room temperature. Divider is flipped (thermistor on bottom, 1 kΩ on top) or the divider bottom is wrong. Confirm the thermistor is on TOP and the FIXED 1 kΩ is on the bottom. Also check that your room really is around 25 °C — a hot summer day above 30 °C brings the thermistor down enough to trigger.
- Fan buzzes but the blade barely turns. You're in the linear region with a cold hand and a knife-edge base current. Cup your palm firmer against the sensor for 10 seconds, or check whether you accidentally used a 470 Ω base resistor instead of 220 Ω.
- Transistor case gets hot. Two suspects: base 220 Ω missing (wire straight from divider to base = deep saturation for warm/hot states, motor stalls at start), or transistor pins reversed. Trace: middle node → 220 Ω → base. Then E-B-C flat-face-forward.
- Diode gets warm. Diode is either wired in series with the motor, or flipped. It goes ACROSS the motor with the banded end (cathode) toward +. Reversed diodes short + through the motor to the collector when the motor runs.
- Fan starts and stops on its own. Ambient temperature drift is crossing the threshold from below. Move away from a heat vent, or lift the sensor off the breadboard so it doesn't heat-soak through the plastic. If the room is close to 30 °C the sensor is right on the edge of firing.
What you're building
Warm-Hand Alarm pointed at the wrong output. Keep everything from that project's sensing half — 10 kΩ NTC thermistor on top of the divider, 1 kΩ on bottom — and swap the LED lane on the collector for a motor plus a flyback diode. Now cupping your palm over the bead starts a gentle breeze on your desk.
If your Warm-Hand Alarm is still on the breadboard, this is a two-minute change: pull the 220 Ω speed-bump and the LED off the output lane, drop in the motor and 1N4001, and shrink the base resistor from 1 kΩ to 220 Ω. Everything left of the transistor stays exactly where it was.
Why is the base resistor different? A motor drinks a lot more than an LED. Same brain, thirstier mouth — you'll do the math below.
Reuse from Level 3 (the "brain")
- Voltage divider: 10 kΩ thermistor on top, 1 kΩ on bottom. At room temperature the middle voltage sits around 0.41 V — well below the transistor's 0.7 V wake-up. Silent at rest.
- 2N2222: turns on when the base sits above ~0.7 V. β ≈ 200.
New at Level 4 (the "mouth")
- DC motor: ~30 Ω coil, spins from ~30 mA up.
- 1N4001 flyback diode: cathode (banded end) toward battery +, anode to the motor's low side (transistor collector). Reverse-biased and invisible while the motor runs; catches the coil's inductive kick the instant the transistor turns off.
- Base resistor drops from 1 kΩ to 220 Ω. A motor at ~150 mA needs more base drive than an 11 mA LED did. Same 2N2222, deeper push.
Do the math
Room, thermistor ≈ 10 kΩ:
Simple divider: 4.5 × (1 ÷ 11) = 0.41 V. Below the 0.7 V wake-up line. Base junction never conducts. Transistor off. Motor idle. The battery only leaks microamps — you can leave this armed all afternoon.
Warm hand, thermistor ≈ 3.6 kΩ:
If nothing loaded the middle node, the divider alone would sit at 4.5 × (1 ÷ 4.6) = 0.98 V. In reality, the base junction pulls it down. Solving KCL at the middle node with the base clamped at 0.7 V:
- V_middle ≈ 0.76 V — just barely above the wake-up line.
- Base current: I_base = (0.76 − 0.7) ÷ 220 Ω ≈ 0.28 mA.
- Collector current: β × I_base ≈ 200 × 0.28 = ≈ 55 mA. The transistor is not in deep saturation — the divider throttles the base drive, so the motor sits in the linear region.
- Motor speed: 55 mA is comfortably above the ~30 mA spin threshold. The blade turns at roughly one-third the speed you saw in Handheld Fan (~143 mA). Enough breeze to feel; nowhere near wall-fan strong.
Hot mug contact, thermistor ≈ 1.5 kΩ:
- Open-circuit middle: 4.5 × (1 ÷ 2.5) = 1.80 V — well above 0.7 V.
- KCL now gives V_middle ≈ 1.0 V, base drive I_base = (1.0 − 0.7) ÷ 220 ≈ 1.4 mA, collector demand β × I_b = 280 mA. But the motor coil only lets 4.5 V ÷ 30 Ω = 150 mA through.
- Transistor saturates. V_CE collapses to ~0.2 V and the motor runs at (4.5 − 0.2) ÷ 30 ≈ 143 mA — full speed, same number Handheld Fan pushes with a plain button.
So the sensor gives you three states you can feel:
| State | Thermistor | Middle | Motor current | What you see |
|---|---|---|---|---|
| Room | 10 kΩ | 0.41 V | ~0 mA | Blade still. |
| Warm palm | 3.6 kΩ | 0.76 V | ~55 mA | Gentle spin. |
| Hot mug | 1.5 kΩ | 1.0 V | ~143 mA | Full speed. |
The bottom-half of that ramp is real analog control. Move your hand closer or press it flatter and the blade speeds up smoothly. Take the hand away and it coasts back to a stop over 30–60 seconds as the bead cools.
Build it
If your Warm-Hand Alarm is still assembled, keep the divider and replace only the output lane: pull the 220 Ω LED speed-bump and the LED; drop in the motor + 1N4001. Then swap the base resistor from 1 kΩ down to 220 Ω — that's the change most likely to bite if you skip it.
Fresh build:
- Power rails: battery + to a top row, battery − to a bottom row.
- The sensing half (from Warm-Hand Alarm): battery + → thermistor → middle row → 1 kΩ → battery −. The thermistor has no polarity. Mount the bead in open air, not tucked under other parts.
- The base link: middle row → 220 Ω → transistor Base. Not the 1 kΩ this time.
- The transistor: flat face toward you, Emitter to − rail, Collector to a fresh row A.
- The motor lane: motor between the + rail and row A. Either lead works — DC motors don't have a polarity, just spin direction.
- Flyback diode across the motor: cathode (banded end) in the + rail row, anode in row A. Exactly the placement from First Spin and Sunrise Fan.
- Power up. In a normal room the blade is still. Cup your palm over the thermistor for 5–10 seconds without touching the leads. The blade should stir, then pick up as your palm warms the bead.
If nothing spins even with a properly warm hand, work backward from the collector: verify the diode's band faces +, then bridge the thermistor's two holes with a plain wire (that fakes "very warm") and should spin the motor. If that works, the thermistor is your suspect — either a bad contact in the breadboard or fingers touching the leads and short-circuiting the sensor.
Where to place it
- Bedside table, palm sensor visible: cup your hand over it before sleep and get a few minutes of breeze while the batteries slowly return the bead to room temperature.
- On the desk under a monitor: rest your palm on the sensor while you're typing and get airflow without a switch.
- Radiator early-warning: thermistor taped to the outside of a cold radiator. The moment the heating kicks on and the surface warms, the fan starts pushing that hot air into the room instead of letting it stack under the ceiling.
The pattern for every level from here on
You've now built the same warm-triggered brain wired to three different mouths:
- Brain: thermistor-on-top divider (warm triggers) →
- LED lane → Warm-Hand Alarm
- LED + buzzer in parallel → Warm-Hand Chorus
- Motor lane → this project
Change the top sensor and the trigger changes: LDR-on-top for light, thermistor-on-top for heat, a microphone amp in Level 8 for sound. Change the collector output and the action changes: LED, buzzer, motor, servo, Arduino pin. Level 5's servo drops into this same collector slot.
🧠 Your challenge
No single right answer. That's the point.
- Cold Fan (flip the sensor). Swap the thermistor and the 1 kΩ so the thermistor sits on the bottom. What does the fan do now? (Answer: it runs in cool air and stops when you warm the sensor — the mirror trick from Sunrise Alarm and Dusk Fan.)
- Predict the fade. Cup your palm gradually — a light hover, then a firm press, then hold it steady. Does the motor jump on or fade in? Why? (Fade in — the thermistor drops smoothly from 10 kΩ toward 3 kΩ, so the middle-node voltage crawls past 0.7 V and the transistor drifts through its analog region.)
- Add a "boost" button. Wire a push button in parallel with the thermistor. Pressing the button shorts the thermistor to 0 Ω, which drives the middle voltage as high as it goes — forcing the fan to full speed regardless of hand temperature. An override for lazy days.
- LED companion. Fan a second lane off the collector: 220 Ω + red LED in parallel with the motor. Now the fan and a small indicator light both fire together on a warm hand — the "one brain, two mouths" trick from Warm-Hand Chorus, but with a motor as the loud mouth. Does the LED steal enough current to weaken the fan? (At warm-palm state the motor sits at ~55 mA linear-region; adding ~10 mA of LED drops the motor to about ~48 mA. Visible but small.)
- Predict, then check: 470 Ω base resistor. Swap the 220 Ω base resistor for a 470 Ω. What happens in the warm-palm state? (Base current drops to about 0.15 mA — collector demand ≈ 30 mA, right at the spin threshold. Motor barely turns.) And in the hot- mug state? (Still saturates — 1.4 mA available even through 470 Ω is more than enough. Full speed.) You've just found where the linear region ends and saturation begins.
- Two-hand fan. Wire a second thermistor with its own 1 kΩ divider, sharing the base line through its own 220 Ω. Now either palm — or both — fires the fan (an OR gate). Sketch this before wiring; think about what the base voltage does when both hands are on.
For grown-ups: safety notes
- Battery-safe throughout — 4.5 V and up to ~150 mA won't shock you.
- The thermistor bead is small and glass or epoxy-coated. Don't bite it, don't scratch the coating with tweezers, and don't press on it hard — the leads are the strong part.
- To test the "hot mug" end of the range, wrap the thermistor in a bit of masking tape and tape it to the outside of a warm mug — never dunk it in liquid. Bare thermistor + water = false readings and a rusted lead. Never touch it to a hot stove, a soldering iron, or the inside of an oven.
- Diode orientation matters. Cathode (banded end) toward battery +. Reversed, the diode conducts continuously with the motor running and will heat up fast. Disconnect and flip it if you see it warming.
- Transistor orientation matters. Flat face toward you, legs left-to-right are Emitter, Base, Collector. Wired backwards, the 2N2222 might work for a while but will run hot in the warm-palm state (linear-region dissipation is real). If the case is uncomfortable to touch, check leg order first.
- The paper fan blade is the fastest-moving part in this project (~2,500–6,000 RPM depending on state). Fast enough to tangle long hair — tie it back — but well below anything that could hurt skin.
- Fingers touching the thermistor's leads warm the bead through metal and can trigger a false fire. Cup the palm above the bead without touching the leads. Same lesson as Warm-Hand Alarm.
- Never wire this to mains. All parts are 4.5 V-safe.
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